In my case, 6 years ago now, the culture was done at our state health dept lab (still standard procedure here). If it is positive for NTM the doc can request sensitivity testing within 14 days. My first pulmonologist didn't request, just treated.

A year later when I didn't clear, the ID doc had a positive culture sent to NJH for testing, it took 4 more weeks and confirmed I was on the correct antibiotics.

Our clinics might now use Quest labs, I haven't asked - they didn't have a big presence here until recently.